Tens of thousands of people are expected to attend Saturday's rally which has been organised by the royalist group, Pitak Siam.

The Internal Security Act, or ISA, enables the government to prevent the use of certain routes or vehicles, impose a curfew, ban gatherings, carry out searches of buildings and censor the media.

Prime Minister Yingluck Shinawatra has appeared on national television to explain the reasons behind the government's decision to impose the security measures.

Ms Yingluck says there is evidence to suggest the protest leaders aim to incite violence to overthrow the government.

The Internal Security Act will be enforced in three districts of Bangkok's historic quarter for nine days.

Correspondent: Delnaaz Irani

Speaker: Yingluck Shinawatra, Thai Prime Minister; Dr John Blaxland, former Australian military diplomat and Thai affairs expert from the Australian National University

BLAXLAND: Pitak Siam is a new manifestation of a phenomenon that we've seen in Thailand in the past. Although they won't necessarily appreciate the connection, essentially they are the new version of what used to be called the Yellow Shirts and they're a Royalist group who are strongly anti-Thaksin Shinawatra, the former prime minister, and, of course, his sister, Yingluck Shinawatra, whose acting essentially in his stead and having won the prime ministership in her own right.

IRANI: As we've just been hearing, the government has put in a lot of very serious precautionary measures. How serious is this anticipated protest this weekend?

BLAXLAND: It's hard to gauge, but it's not unreasonable, I believe to invoke the Internal Security Act, given the past behaviour of groups that have a Royalist affiliation, such as the Yellow Shirts so-called People's Alliance for Democracy has it's called or the PAD.

Remember back when they were protesting, they were involved in closing down the major airport of Bangkok, Suvarnabhumi Airport. They were involved in occupying a number of significant government buildings, so there are some concerns that they might actually try to seize some buildings again this time or come up with another plan, so there are legitimate concerns about what they might get up to. And General Boonlert Kaewprasit, the head of the Pitak Siam, which actually translates as Defend Thailand. He's actually a pugilist himself, he's head of the Thai Boxing Association, so he's a fairly combative sort of a guy.

IRANI: And this group is really sending a lot of mixed messages about just how far they're willing to go. Do we know how much support they have, especially among key powerbrokers, like the military?

BLAXLAND: Well, that's a very interesting point, because Thailand is a riven society unfortunately at the moment. There are many in Bangkok who really don't appreciate the current government and it's very much polarised. So the military which has traditionally not been all that sympathetic to in fact, some would argue hostile toward the Shinawatra family. They are in a position where they don't want to be seen to be connected to closely with this call, this implication ?? of the Internal Security Act.

This is something they would rather see the police handle. The police is the institution that's more closely associated with the Shinawatra family and understandably so, given that Thaksin himself is a former police officer. The military, the army in particular, the most prominent arm of the military in Thailand is it's going to be very reluctant to be involved here and understandably so. They're own sympathies lie with the Royalist camp, even though General Prayuth Chan-ocha, the Commander in Chief of the Royal Thai Army, is publicly very supportive of the government. The sympathies of the military itself tend to be not that sympathetic with Yingluck Shinawatra's government. But they're trying to toe a line, so it's a very tricky position that they're in with the invocation of the ISA to know how far they'll go. But if we look at previous instances, we know that the military has tended to be fairly reserved, when a Shinawatra-aligned government has been in office. So I expect we'll probably see a bit more of that this time around.

IRANI: So Dr. Blaxland, do you think that there is really any danger that the government could fall because of this?

BLAXLAND: Look, there is always a risk of that, but that could happen through the censure debate as well. My sense is that that's not all that likely at this stage.

The military has had its fingers burnt essentially from the experience of staging a coup in 2006 and all the indications that I've seen are that the military's very reluctant to go there again, even though a group like Pitak Siam might be very strongly encouraging them to go down that path.

General Prayuth knows that's a no-win situation. There's nothing really to be gained, because the factors that are polarising Thai society today won't go away simply by staging another coup.

IRANI: And how do these protests fit in with this no-confidence vote that's due to come next week in Thai Parliament?

BLAXLAND: Well, I mean the bottom line is that the Pitak Siam group knows that the numbers aren't there for the censure debate to win, to overturn the government, so their going through the motions in the censure debate. But the rally is hopefully going to add some momentum, add a degree of gravitas to the occasion and hopefully maybe turn some of those wavering politicians.

My sense, however, is that they're over-estimating what they can pull off, they're overplaying their hand.

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